1. Field of the Invention
The present invention relates to an external cavity laser type light source having a wavelength selection element.
2. Description of the Related Art
Referring to FIGS. 5 and 6, an external cavity type semiconductor laser light source will be described as an example of a conventional external cavity type light source. (A semiconductor laser will be referred to as LD in this specification hereinafter.)
FIG. 5 is a view showing an example of the arrangement of the conventional external cavity type LD light source. FIG. 6 is a graph showing a spectrum of output light of the external cavity type LD light source shown in FIG. 5.
In the conventional example shown in FIG. 5, reference numeral 200 designates an LD, and reference numerals 201 and 202 designate end surfaces of the LD. Usually, an antireflection coating is provided on one end surface 201 of the external cavity type LD light source so as to prevent the occurrence of Fabry-Perot resonance on both end surfaces of the LD.
After light is emitted from the end surface 201 on which the antireflection coating is provided, it is made to be parallel light by a lens 210 and then incident upon a diffraction grating 220 which is a wavelength selection element. In this connection, a band-pass filter may be used as the wavelength selection element.
After the wavelength of light is selected by the diffraction grating 220, the direction of light is changed by an angle of 180xc2x0. Then, light is condensed by the lens 210 and returned to the LD 200. In this case, an external cavity or resonator is composed of the end surface 202 of the LD 200 and the diffraction grating 220, and laser beam oscillation can be conducted by the resonator.
On the other hand, after light is emitted from the end surface 202 on which no antireflection coating is provided, it is made to be parallel light by a lens 230 and passes through an optical isolator 240. Then, light is condensed by a lens 250 and taken out from an optical fiber 260 as output light.
As shown in FIG. 6, a single wavelength selected by the diffraction grating 220 is ruling in the wavelength components of the output light. However, in addition to the single wavelength selected by the diffraction grating 220, the wavelength components of the output light contain spontaneous emission light, the wavelength band of which is wide, which is directly emitted from the LD 200 onto the lens 230.
However, the following problems may be encountered in the above external cavity type LD light source. As described above, when the above external cavity type LD light source is used, output light contains laser beam, the wavelength of which has been selected by the wavelength selection element, and spontaneous emission light which has been directly emitted from the light emitting element. Therefore, when the characteristic of an optical filter or the like is measured by combining the external cavity type LD light source with a power meter, it is impossible to conduct measurement accurately. Especially, in the case of measurement conducted on a notch type filter, the above problem becomes remarkable.
In general, a ratio of an intensity of power of the laser beam of a single wavelength to an intensity of power of spontaneous emission light of a wide wavelength band is referred to as a side mode suppression ratio, which is approximately 40 dB in the conventional example described before.
An object of the present invention is to provide an external cavity laser type light source, the wavelength purity of output light of which is very high, wherein an unnecessary spontaneous emission light component except for a selected wavelength component is cut off from the output light.
To achieve the above object, the invention provides an external cavity laser type light source comprising: a light emitting element; a wavelength selection element for selecting a wavelength of light emitted from the light emitting element and for returning light to the light emitting element; and an optical branch element arranged between the light emitting element and the wavelength selection element, wherein selected light sent from the wavelength selection element is branched by the optical branch element, and one branched component is taken out as output light.
According to the invention, the optical branch element is arranged between the light emitting element and the wavelength selection element, and light selected by the wavelength selection element is branched into two beams of light by the optical branch element. Then, one of the branched beams of light is taken out as output light. Due to the foregoing, an unnecessary spontaneous emission light component, the wavelength of which is different from the selected wavelength, can be cut off from the output light, so that the output light of highly pure wavelength can be obtained.
Here, examples of the light emitting element are a semiconductor laser element, a solid state laser element, a liquid laser element and a gas laser element. An example of the optical branch element is an unpolarized light beam splitter. Examples of the wavelength selection element are a filter type wavelength selection element and a diffraction grating type wavelength selection element.
Further, the invention provides an external cavity laser type light source comprising: a light emitting element; a diffraction grating for selecting a wavelength of light emitted from the light emitting element and for returning light to the light emitting element; and a mirror for reflecting light, wavelength of which has been selected by the diffraction grating, so that a direction of light is changed by an angle of 180xc2x0 and light is made to be incident upon the diffraction grating again, the mirror being a multiple-surface reflecting mirror having at least two reflecting surfaces, angles of which are different from each other.
According to the invention, there is provided the mirror for reflecting light, the wavelength of which has been selected by the diffraction grating, so that the direction of light can be changed by an angle of 180xc2x0 and the light can be made to be incident upon the diffraction grating again. Therefore, a portion of light, the wavelength of which has been selected in the diffraction grating, is returned to the light emitting element. Since the mirror is a multiple-surface reflecting mirror having at least two reflecting surfaces, the angles of which are different from each other, a portion of light, the wavelength of which has been selected in the diffraction grating, proceeds in a direction different from that of the optical axis of the light emitting element.
Since light, the wavelength of which has been selected in the diffraction grating, is branched into two beams of light by the mirror, one of the beams of light can be taken out as output light. That is, output light, the wavelength purity of which is very high, from which an unnecessary spontaneous emission light component except for the selected wavelength component is removed, can be obtained.